1. Field of the Invention
This invention relates to a still image pickup apparatus and more particularly to a still image pickup apparatus of the type arranged to use both the removal of electric charges from an image sensor and a mechanical light-shielding means for controlling exposure time.
2. Description of the Related Art
In the field of still video cameras which have recently become saliently popular, a length of time for which an image sensor is to be illuminated with an optical image of an object is generally controlled by means of a mechanical shutter. This arrangement may be regarded as simply replacing a film used for a silver-halide camera with the image sensor. The aperture of the lens stop of the still video camera is controlled by means of an iris (diaphragm) which is disposed within the lens.
In another example of the still video cameras, a turret stop is used for the aperture control while a mechanical shutter is used for the exposure time control. Further, some of the cameras of this kind is arranged to control the exposure time (accumulation time) by means of an electronic shutter, leaving the mechanical shutter in a full open state, in the event of continuously taking a series of shots at a high speed.
However, the image sensor of the still image pickup apparatus such as the still video camera mentioned above gives a narrower latitude than the camera of the kind using a silver halide film. Accurate control over the aperture and the exposure time is not easy and thus calls for an expensive arrangement, which results in an increase in cost.
This problem in respect to the cost increase may be mitigated by arranging the aperture to be variable stepwise instead of continuous. However, this arrangement still presents a problem, because the camera must be equipped both with the shutter and the iris. Besides, the shutter control accuracy attainable by this arrangement is insufficient.
Further, the cost increase problem may seem to be mitigable also by lessening the number of parts by arranging one set of blades to be used both as an iris and as a shutter. However, that solution still presents a problem in respect to an operation characteristic, because: In order to ensure accurate control over an aperture defined by the blades, these blades must be operated at a low speed, which makes it hardly possible to use them as a high-speed shutter.
This shortcoming not only makes a high-speed shutter hardly obtainable but also narrowly restricts an object luminance range within which a flash device is usable for the so-called fill-in flash shooting for preventing, with the light of the flash device, an object having a high luminance background from giving a darkly blank image. The shortcoming thus prevents a back-light shot from being adequately corrected.
Further, in cases where a motor is used for driving the blades, the aperture is stabilized by bringing the blades to a stop either by cutting off power supply to the blade opening motor or by reversely energizing the motor. The speed of the blades varies according to a voltage applied to the motor. Therefore, in bringing the blades to a stop at an aperture position, an error which takes place before the blades actually come to a stop fluctuates for the same aperture position. Fluctuations in voltage of the power supply thus make the aperture of the iris unstable. To avoid the instability, it is necessary either to calculate an apposite exposure time by measuring once again the light passing through the actual aperture after stabilization of the iris aperture or to stabilize the iris aperture by providing the camera with a means for stabilizing the voltage to be applied to the motor. Such arrangement results in a complex system.
U.S. patent application Ser. No. 767,512, filed on Sep. 30, 1991, has disclosed an image pickup device which solves the above-stated problem. The image pickup device comprises image pickup means arranged to be capable of clearing and reading image information for controlling accumulation time and having a first reading mode in which the information is read out once from effective picture elements and a second reading mode in which the information is read out divisionally from effective picture elements by carrying out the reading action a plurality of times, and optical light-shielding means. The accumulation time of the image pickup means is set through the image information clearing action of the image pickup means and the light-shielding action of the optical light-shielding means when the image pickup means is operating in the second reading mode, so that exposure control can be accurately accomplished in a still video camera or the like without any complex arrangement.
According to the image pickup device disclosed, the light-shielding action of the optical, or mechanical, light-shielding means begins after the photoelectron removing action of the image sensor. However, a length of time required for the light-shielding action of the mechanical light-shielding means from the start to the end of the light-shielding action is such a length that is hardly ignorable for the exposure time. Therefore, the exposure time cannot be shortened to a length less than the time necessary for the action of the mechanical light-shielding means. Hence, an over-exposure tends to occur in taking a shot of a high luminance object. In taking a shot of a fast moving object, a still image is hardly obtainable and the shot results in a blurred image.